PMK (pairwise master key) Caching -- this is "fast roam back." In
other words, the AP just caches PMKs and allows clients that have been
connected previously to roam back to it without full 802.1X auth.

OKC (Opportunistic Key Caching) -- this actively reuses PMKs from
one AP with other APs in the same zone. So when a client roams to a new
AP, it can reference the prior PMK with a PMKID and thus bypass full
802.1X.